Electret condenser microphone elements are well known in the art and used in a variety of applications, for example landline and cellular telephones, broadcast and recording systems, communication headsets, and computer microphones. Such microphone elements can be designed to be either directional or omnidirectional, depending upon the desired application and performance requirements.
The designs for directional and omnidirectional microphone elements can differ in a variety of ways. One of the principal distinguishing design features between these two microphone designs is in the placement of the sound port(s), also referred to herein as an acoustic aperture(s). In a directional microphone, there are at least two spatially separated sound ports, this feature leading to a decreased pick-up of low frequency background ambient acoustical noise. In contrast, in an omnidirectional microphone element the sound port is located in a single spatial position, even if the microphone includes multiple ports. As a result of this design, omnidirectional microphone elements are less susceptible to the pick-up of wind noise than directional microphone elements. Wind or turbulent-type noise is present any time air is flowing past the microphone aperture(s), such as in automotive environments or from a fan.
Although in general omnidirectional microphone elements are less susceptible to wind noise than directional microphone elements, their lack of spatial discrimination can allow decreased signal quality when used in environments in which the primary audio signal source, e.g., the intended speaker, is surrounded by a high level of background ambient noise (e.g., traffic noise, machinery including engine and HVAC noise, and background vocal noise). Additionally, as omnidirectional microphone elements have a generally flat frequency response from about 50 or 100 Hz to about 10 kHz, they are further prone to picking up background ambient noise since the sound pressure levels associated with typical background ambient noise increase at lower frequencies. In contrast, background ambient noise is less problematic for directional microphone elements which exhibit a natural response roll-off at lower frequencies, and importantly because they spatially discriminate against acoustical noise from selected directions.
FIG. 1 is a cross-sectional view of an exemplary configuration of a conventional omnidirectional electret microphone element 100. Microphone 100 is comprised of an electrically conductive, cylindrical casing 101. The front face 103 of one end portion of casing 101 includes one or more, substantially co-located, acoustic apertures 105. An electrode plate 107 with one or more secondary acoustic apertures 109 fits against the inner surface of front portion 103 of casing 101. An electret material 111 is deposited on, or otherwise applied to, the inner surface of electrode 107. A metallized diaphragm 113 is separated from electret material layer 111 by an electrically insulating spacer 115.
A circuit board 117 fits within, and covers, the casing opening located at the distal end opposite front face 103. One or more signal processing elements 119 (e.g., a field effect transistor or FET) are attached to circuit board 117 and contained within casing 101 as shown. Electrode patterns on circuit board 117, represented by raised contact regions 118, are used in conjunction with electrically conductive casing 101 to couple signal processing element 119 to electrode plate 107. Metallized diaphragm 113 is coupled to signal processing element 119 via an electrically conductive spacer 121 and a raised contact region 123 located on the bottom surface of circuit board 117. Spacer 121 is typically ring-shaped. A second electrically insulating spacer 125, typically ring-shaped, is used to prevent shorting of spacer 121 to casing 101 as well as insuring that spacer 121 is properly positioned relative to contact region 123. End edge portion 127 of casing 101 is folded over and crimped, thereby compressing circuit board 117, spacer 121, and metallized diaphragm 113 against each other and holding the individual components in place. Solder bumps 129 are used to electrically couple the microphone element to the intended device (i.e., cell phone, camcorder, etc.).
As known by those of skill in the art, there are numerous possible configurations for a conventional omnidirectional electret condenser microphone element. The microphone element described above relative to FIG. 1 is but one such configuration, generally referred to as an inverted back-electret arrangement. Another exemplary prior art arrangement, referred to as a back-electret arrangement, reverses the positions of elements 107, 111, 115, and 113, placing electrode 107 toward the back of the structure. In such a configuration, electrode 107 is usually called the backplate. In yet another exemplary prior art arrangement, referred to as a foil-electret arrangement, the electret material layer is deposited on the diaphragm instead of being placed on the electrode. In both alternate configurations briefly described above, other changes to the structure are necessary.
In at least one conventional omnidirectional electret condenser microphone element known to the inventors, means are provided to achieve quasi-static pressure equalization between internal microphone volume 131 and the ambient environment. It will be appreciated that pressure equalization means can be specifically designed into the element, for example utilizing a leakage passageway as described more fully below, or by taking advantage of the normal mismatch between components within the microphone assembly. Quasi-static pressure equalization is often desired to avoid potentially damaging the diaphragm when the microphone is subjected to sudden and major pressure changes, for example those commonly encountered during air shipment. At the same time, and as known by those of skill in the art, the means used to provide pressure equalization must allow only minor air leakage between the ambient environment and volume 131, otherwise the microphone element will fail to operate properly and to provide the desired electro-acoustic response. In a typical omnidirectional electret condenser microphone element utilizing pressure equalization means, the leakage passageway is small enough that only frequencies below the audio band, for example near 5-10 Hz, are affected.
FIG. 2 illustrates a conventional omnidirectional electret condenser microphone element similar to the microphone element shown in FIG. 1, with the addition of a pair of pressure equalization leakage passageways that have been designed into the assembly. Such pressure equalization leakage passageways are known in the art. As previously noted, normal component mismatch within the assembly is often used to accomplish a similarly sized air leak.
As shown in FIG. 2, microphone element 200 includes a pair of small passageways 201 and 203 that allow air to leak around circuit board 117, thereby coupling the ambient environment to acoustic volume 131. Passageway 201 is formed by including a slot between crimped casing end portion 127 and the upper surface of circuit board 117. This slot, referred to in FIG. 2 by passageway 201, is formed by including an interruption within contact region 118 so that when edge portion 127 is crimped against the circuit board an air passageway remains. Similarly, passageway 203 is formed by including an interruption within contact region 123 that remains after circuit board 117 is pressed against spacer 121. Accordingly passageways 201 and 203, in combination with the use of a circuit board 117 that has a slightly smaller outside diameter than the inside diameter of casing 101, allows air to leak around the circuit board, thereby achieving the desired pressure equalization.
FIGS. 3 and 4 illustrate another observed modification of the electret condenser microphone element of FIG. 1. Microphone 300, which exhibits the typical, substantially flat response of an omnidirectional microphone, includes a notch 301 in the front surface 401 of casing 303.
What is needed in the art is an omnidirectional electret condenser microphone element, such as the conventional unit described above, but in which the design has been modified to reduce the pick-up of background noise, thereby providing an enhanced signal-to-acoustic background ambient noise ratio. The present invention provides a means for achieving such a microphone.